User of device, a feature in film (9)
I believe the answer is:
machinist
'user of device' is the definition.
(someone using a machine)
'a feature in film' is the wordplay.
'feature' becomes 'chin' (chin is a kind of feature).
'in' means one lot of letters goes inside another.
'film' becomes 'mist' (I've seen this before**).
'a'+'chin'='achin'
'achin' put inside 'mist' is 'MACHINIST'.
(Other definitions for machinist that I've seen before include "Engineer" , "tool operator" , "factory worker, perhaps" , "Eg, lathe operator" , "One working on a device" .)